The present invention relates to a method of soldering two members to each other, and more specifically a method of soldering two members to each other so that solder used for bonding does not run over a given region. Also, it relates to a method of manufacturing a semiconductor device by soldering.
Soldering has been generally adopted as a method of mounting a component on a circuit pattern on an insulating board. Also, soldering has been adopted as a method of bonding an insulating board to a cooling plate. Soldering is performed by applying a solder paste to opposing faces of two members to be bonded to each other or putting a solder sheet between the faces and then melting the solder paste or sheet.
For such process, a method of using a solder resist film has been known as a method of bonding two members in position, which is disclosed in, e.g., JP-A-2004-47848. The solder resist is a resin composition. A solder resist film is formed in a given pattern on the surface of one of two members to be bonded. As a portion on which the solder resist film lies is never wetted with solder, melted solder can be made to stay on a given soldering region on which no solder resist is formed, and therefore it is possible to prevent the solder from adhering to a region other than the soldering region.
FIGS. 7A and 7B are illustrations showing an important portion of a semiconductor device manufactured using a solder resist. Of the drawings, FIG. 7A is a top view, and FIG. 7B is a sectional view of the important portion taken along the broken line shown in FIG. 7A. In the drawings, the reference numeral 1 denotes a cooling base plate made of copper; the numeral 2 denotes a ceramic plate; the numeral 3 denotes a copper pattern formed on a front face of the ceramic plate; and the numeral 4 denotes a copper pattern formed on a rear face of the ceramic plate. The ceramic plate 2 and copper patterns 3 and 4 constitute an insulating board 5. The numeral 6 denotes a semiconductor element, such as an IGBT (Insulated Gate Bipolar Transistor), and a FWD (Free Wheeling Diode). The cooling base plate 1 and the copper pattern 4 on the rear face of the insulating board 5 are bonded by solder 7. Also, the copper pattern 3 on the front face of the insulating board 5 and the semiconductor element 6 are bonded by the solder 7.
To prevent the solder 7 from running over a given soldering region during soldering, a solder resist film 10 is formed on an outer peripheral portion surrounding the soldering region on the surface of the cooling base 1. The solder resist film 10 does not surround the outer periphery of the insulating board 5 completely as in FIG. 7A. This is because the insulating board 5 can be prevented from being displaced as the solder 7 is melted and positioned in place as long as each corner portion of the insulating board 5 is surrounded by the solder resist film 10. Further, a portion between two insulating boards 5 is covered with the solder resist film 10. The reason for this is because adhesion of solder to this portion deteriorates the withstand voltage between the insulating boards 5, and therefore a measure to prevent deterioration in the withstand voltage is taken.
FIGS. 8A-8C are illustrations that help explain the process of forming the solder resist film 10 on the cooling base plate 1. Of the drawings, FIG. 8A shows the cooling base plate 1 prior to formation of the solder resist film 10. FIG. 8B shows a mask 11 for screen-printing the solder resist film in a given pattern on the cooling base plate 1. FIG. 8C shows the cooling base plate 1 with the solder resist film 10 formed thereon.
Now, the process of forming the solder resist film 10 will be described. First, the cooling base plate 1 without the solder resist film 10 is aligned with a stage (not shown), and then put on the stage. Second, the mask as shown in FIG. 8B is put on the cooling base plate 1 after the alignment thereof. Then, some solder resist (i.e. resin ink, not shown) is dropped on the mask 11. The solder resist is printed on a surface of the cooling base plate 1 through fine holes formed in the mask 11 according to a given pattern.
Thereafter the mask 11 is removed and the solder resist is dried and hardened. The resin ink of the solder resist is made of a resin which can be hardened by heating or ultraviolet radiation, and therefore a step to harden the solder resist by heating or ultraviolet radiation is needed after application of the ink onto the cooling base plate 1. The time required between the application of the solder resist and hardening thereof is a few hours. An example of the method as described above is disclosed in JP-A-2004-47848.
The conventional solder resist film as described above is a resin composition. The assembly processes for semiconductor devices include a heating process in soldering, and a chemical cleaning process for removing the residue of flux contained in solder. A solder resist film having a resin composition has had a problem in that the solder resist film is exfoliated in the process of manufacturing semiconductor devices because of the influences of a thermal hysteresis in the heating process and chemicals used for washing. Also, it takes much time to prepare a cooling base plate with a solder resist film 10 formed thereon because the steps for drying and hardening the solder resist are required after application of the solder resist (resin ink).
Further, to print a solder resist by means of screen-printing, masks 11 corresponding in number to types of patterns of the solder resist film 10 are required. It is difficult to form a fine pattern by use of the mask 11. Moreover, even an insignificant change in design of a semiconductor device will lead to the change in the pattern of the solder resist film and increase the types of patterns. Also, it is necessary to form a mask 11 each time the pattern is changed or increased in number, which becomes a factor to raise the manufacturing cost.
Further, a mask for screen-printing has a fine through-hole through which a solder resist (resin ink) flows, and therefore if the mask is left as it is after screen-printing, the solder resist will solidify in the through-hole, and block up the through-hole. Hence, maintenance is required to use the mask 11 repeatedly, such as cleaning the mask with an organic solvent each time the step of printing is completed.
In view of the above, the invention aims to solve the problems involved with a conventional solder resist film having a resin composition.